1. Technical Field of the Invention
The present invention relates to the decoding of radio transmission channels carrying coded digital information. More precisely, the invention relates to the acquisition of the carrier frequency of a signal modulated with a quadrature single-carrier continuous modulation, for example, a quadrature amplitude modulation known as QAM.
2. Description of Related Art
Solutions currently used to carry out the acquisition and monitoring of the carrier frequency are based on phase-locked loops.
More precisely, a first category of solution uses a phase-locked loop having a wide frequency locking range for the acquisition of the carrier frequency, then a narrower frequency locking range when monitoring the carrier frequency. However, such a solution has a disadvantage in that the acquisition may fail with an input signal of poor quality.
A second category of solution uses a phase-locked loop having a narrow frequency locking range in order to be able to carry out the acquisition of a poor quality signal and in parallel uses a frequency scanning algorithm in order to acquire a signal exhibiting a significant frequency shift from the carrier frequency. However, this second solution has a disadvantage of requiring considerable locking time.
A need accordingly exists in the art to address the foregoing disadvantages.
More specifically, a need exists to be able to estimate and correct significant frequency shifts, for example of the order of hundreds of kHz even with a poor quality input signal, for example exhibiting a signal-to-noise ratio of the order of 20 dB for a 64 QAM modulation, and doing so for a short time, for example of the order of a hundred milliseconds.